1. Technical Field
The invention is directed to Light Emitting Diodes (LEDs) and, in particular, to LEDs that emit light in a pattern.
2. Art Background
Flat panel displays containing light emitting diodes are ubiquitous features of many products. Because of the need to minimize the manufacturing cost of most products, inexpensive ways to manufacture flat panel displays are of considerable interest. As noted in Lidzey, D. G., et al., "Photoprocessed and micropatterned conjugated polymer LEDs," Synthetic Metals, Vol. 82, pp. 141-148 (1996), organic materials have been investigated for use as the emissive layers in LEDs because large-area devices can be made cheaply and easily using such materials. Also, a greater variety of emission colors is obtained when organic emissive layers are used instead of inorganic emissive layers. LEDs with organic emissive layers have a greater electrical efficiency than comparable LEDs with an inorganic emissive layer.
LEDs are generally formed on transparent substrates such as glass or plastic. A light emitting material is sandwiched between an anode formed on the substrate and a cathode. When current is supplied to the anode, electrons and holes recombine in the light-emitting material sandwiched between the anode and the cathode. As a result of this recombination, light emits from the light-emitting material and through the transparent substrate.
One use for LEDs is in displays having a fixed pattern. In such displays, there are at least two areas of contrast when the display is on. The areas of contrast (e.g. light and dark) provide a desired picture (e.g., a logo) or message (e.g. an "EXIT" sign). Such a patterned array of LEDs is described in the previously mentioned Lidzey et al. reference which was mentioned previously. A patterned cathode is formed over an emissive layer (poly(2,5-dialkoxy-p-phenylenevinylene). When a voltage is applied to the ITO anode, light is emitted in a pattern that corresponds to the cathode pattern, because light is only emitted from those portions of the emissive layer sandwiched between the anode and the cathode. A patterned display is also obtained by patterning the anode instead of the cathode.
However, there are certain limitations on the patterns that can be obtained by patterning the anode or the cathode. For example, a simple pattern such as the letter "O" is not easily obtained by patterning the cathode. This is because the mask used to form the pattern must be one integral unit. The letter "O" requires complete physical separation between the portion of the mask inside the "O" from the portion outside the "O." Such a complete physical separation cannot be obtained in a single unit mask. There must be some physical connection between the portion of the mask inside the "O" and the portion of the mask outside the "O." Furthermore, the expedients used to pattern the cathode in the manner described in Lidzey et al. degrade the organic emissive layer underlying the cathode.
Different restrictions are placed on a patterned anode such as indium tin oxide. For example, the conductivity of ITO is reduced when patterned into narrow lines. Therefore, the brightness of the display is not evenly distributed if a narrow portion of ITO is required by the pattern. Furthermore, the ITO must be electrically interconnected and therefore a pattern that is not continuous is not practicable.
In response to the limitations imposed by patterning anodes and cathodes, Renak, M., et al., Microlithographic Process for Patterning Conjugated Emissive Polymers," Advanced Materials, Vol. 9, No. 5, pp. 392-395 (1995) describes a patterned LED display in which the electron emissive layer (poly(p-phenylenevinylene)) is patterned. Renak et al. describes a device in which the patterned layer of poly(p-phenylenevinylene) (PPV) is formed over an ITO layer. An electron transport layer was cast over the PPV layer. A cathode was formed over the electron transport layer. The electron transport layer is present to prevent direct electrical contact between the ITO anode and the cathode.
When a voltage is applied to the ITO of the device described in Renak et al., light is emitted from the patterned PPV layer in the pattern of the PPV layer. However, the approach does not afford much flexibility, as the only contrast provided by such a display is the contrast between the PPV area of the display (which emits light when the device is on) and the non-PPV area of the display (which does not emit light even when the device is on). Thus the basis for contrast in such a display is basically either on or off. Furthermore, Renak et al requires the use of light-emitting polymers that are also photosensitive in order to pattern the light-emitting layer. Thus, the choices for the lightemitting material for the Renak et al. device are extremely limited.
A display that provides the potential for a greater variety of visual contrast, yet does not require that either the anode or the cathode be patterned, is desired.